1. Field of the Invention
The present invention relates to an interference cancellation circuit, and more particularly to an interference cancellation circuit capable of performing interference cancellation of an incoming signal. The incoming signal includes a transmitted signal and at least one accompanying interference signals at the frond end of a receiver for wireless communication systems. The accompanying interference signals in the incoming signal are usually stronger than the transmitted signal so it may saturate the LNA (Low Noise Amplifier) and thereby ruin the transmitted signal.
2. Description of the Related Art
To describe the related art of the present invention, the relation between a LNA and the other parts of a typical wireless receiver shall be introduced first. Please refer to FIG. 1a, which shows the architecture of a typical wireless receiver. As shown in FIG. 1a, the typical wireless receiver at least comprises a gain-controllable LNA (Low Noise Amplifier) 101, a power detector 102, a mixer 103, a local oscillator 104, a mixer 105, a mixer 106, a quadrature-phases type local oscillator 107, a complex filter 108, a variable-gain amplifier 109, an automatic gain controller 110 and an analog to digital converter 111.
Since the mixer 103, local oscillator 104, mixer 105, mixer 106, quadrature-phases type local oscillator 107, complex filter 108, variable-gain amplifier 109, automatic gain controller 110, and analog to digital converter 111 are well known and not the focus of the present invention, therefore they will not be further addressed hereafter.
The gain-controllable LNA 101 is used for amplifying an incoming signal comprising a transmitted signal and some possible interference signals. The power detector 102 is incorporated in around the gain-controllable LNA 101 to adjust the gain of the gain-controllable LNA 101 in response to the power of the incoming signal. If there come the possible interference signals, which are usually stronger than the transmitted signal in the incoming signal, the power detector will detect a larger power and then send a signal to lower the gain of the gain-controllable LNA 101 to prevent saturation. However, the noise figure will be increased due to the lowered gain of the gain-controllable LNA 101. The noise budget of the typical wireless receiver is usually so tight that the increased noise figure of the output of the gain-controllable LNA 101 will encroach on the noise figures of the other parts of the receiver. Therefore, there is a demand to lower down the noise figure of the LNA to yield room for the noise figure of the other parts of the receiver.
One solution to eliminate the possible interference signals is to insert a SAW filter before the LNA. Please refer to FIG. 2, which shows a block diagram of a prior art for eliminating the interference signals of an incoming signal. As shown in FIG. 2, the prior art comprises a SAW filter 201, a gain-controllable LNA 202, a power detector 203 and a mixer 204.
The SAW filter 201 is used to filter out the interference signals. However, the adoption of the SAW filter 201 will occupy additional board area in the circuit, add more component cost, and cause noise figure degradation due to a SAW filter's insertion loss.
Another solution is to implement a second path comprising a notch filter to retain only the interference signals, and then subtract the output of the second path from the output of the LNA. Please refer to FIG. 3, which shows a block diagram of another prior art named feed-forward technique. As shown in FIG. 3, the feed-forward technique comprises a LNA 301, an RF notch filter 302, a subtractor 303 and an amplifier 304.
The RF notch filter 302 is used for filtering out the transmitted signal and retaining the interference signals intended to be mutually cancelled out with the interference signals of the other path at the subtractor 303. However, the phase shifts of the two paths are different due to unsymmetrical structures, so the effect of interference cancellation is limited. Besides, a high quality RF filter is physically difficult to be implemented on-chip and the high insertion loss of the on-chip RF filter if at all will also significantly degrade the receiver sensitivity.
Still another solution is to implement a second path including two mixers and a HPF (High Pass Filter) to retain only the interference signals, and then subtract the output of the second path from the output of the LNA. Please refer to FIG. 4, which shows a block diagram of still another prior art technology named translational-loop technique. As shown in FIG. 4, the translational-loop technique comprises a LNA 401, a mixer 402, a HPF (High Pass Filter) 403, a mixer 404 and a subtractor 405.
The combination of the mixer 402, HPF 403 and mixer 404 acts as an RF notch filter used to filter out the transmitted signal and retain the interference signals intended to be mutually cancelled out with the interference signals of the other path at the subtractor 405. However, the mixer 402 and mixer 404 will produce additional images, and the phase shifts of the two paths are different due to unsymmetrical structures, so the effect of interference cancellation is limited. Therefore, there needs a technology able to offer superior performance in interference cancellation at the front end of the wireless receiver with concise structure and minimum cost.